Its only strange if you think all software will be perfect, the developers (authors) of these style/themes which aren't difficult to do, mainly being an adaption from a basic or other style set, already existing..
--won't likely be tested against all environments, that they are offered for.. (desktops, that is)
So we could expect different writers of these themes/styles to have or not have better coding (or tighter coding)
Abut the general issue, unless a theme is causing some system slowdowns but doesn't have any other artifacts, such as quirky display results, then I generally ignore it
Cpu percentage doesn't bother me if the overall system is AOK! (apps run fine, and displays are fine..)
--if it did, I would be in an unending quest to wring every ounce of performance from my system: that I think is a waste of my time